I have something for you to try. When you are ready to reassemble the fuel system and run the tractor, install the elbow as normal. Do not use teflon tape or any other sealer prior to assembly. Clean the threads and crack well with carb cleaner. After assembly
, but prior to introducing fuel, apply some Permatex green threadlocker to the threads and the crack. Allow it to cure according to the label.
The threadlocker is gasoline resistant and should seal everything properly. The good news is this solution is not permanent and will not further impact the carb. You can still JB weld or solder the crack if this doesn't hold. And it's not expensive.